1. FIELD OF THE INVENTION
This invention relates to an electromagnetic force valve driving apparatus for opening and closing the suction/exhaust valve of an engine by an electromagnetic force produced by an electromagnet.
2. DESCRIPTION OF THE PRIOR ART
In one example of a conventional drive apparatus for opening and closing a suction/exhaust valve, a cam shaft on which cams for suction and exhaust are disposed is provided on the upper portion of the engine or on one side face thereof. A crankshaft, which is the rotary shaft of the engine, and the cam shaft are connected by rotary transmission means such as a belt, and the cam shaft is driven rotatively in synchronism with the rotational phase of the engine.
The cam face of the cam shaft and the axial end face of the valve are connected via a link mechanism such as a rocker arm or pushing rod. The suction/exhaust valve, which is biased in the closing direction at all times by a valve spring, is driven in the opening direction by the link mechanism which acts to push the axial end face of the valve.
This conventional drive apparatus for opening and closing the suction/exhaust valve results in a large-size engine because the cam shaft and link mechanism must be added to the engine.
Furthermore, since the cam shaft and link mechanism are driven by the output shaft of the engine, some of the engine output is consumed by frictional resistance when the cam shaft and link mechanism are driven. This diminishes the effective output of the engine.
Further, the actuation timing of the suction/discharge valve cannot be altered during engine operation. Since the valve actuation timing is adjusted in conformity with the rotational speed of the engine, engine and output and efficiency decline when the engine is running at an rpm different from the prescribed rpm.
In order to solve the foregoing problems, an apparatus for driving a suction/exhaust valve by an attractive force acting between a movable magnetic pole connected to the suction/exhaust valve and a magnetic pole of a fixed electromagnet has been disclosed in Japanese Patent Application Laid-Open (KOKAI) Nos. 58-183805 and 61-76713.
In this apparatus, the distance between the magnetic pole of the electromagnet and the movable magnetic pole is maximum at the moment the attractive force starts acting upon the movable magnetic pole. Consequently, the attractive force between the magnetic pole of the electromagnet and the movable magnetic pole is minimum at this time. Accordingly, acceleration of the movable magnetic pole immediately after it starts moving is low, and therefore the size of the opening of the valve operatively associated with the movable magnetic pole is small.
Further, since the distance between the electromagnet and the movable magnetic pole is large even in a case where a braking force is applied to the movable magnetic pole just prior to valve seating in order to mitigate shock when the valve is seated, as described in the specification of Japanese Patent Application Laid-Open No. 61-76713, the braking force is too small to reduce the seating shock sufficiently.